I totally agree with that point of they can't have a set of hard and fast rules or the show just doesn't work. Like I don't give a shit that the chronovores or whatever they were called don't show up every time there's a paradox. I don't care that sometimes a paradox will rip open the universe and need a TARDIS to sustain one without that happening and other times they just have 10 paradoxes in a row to make a fun episode. But what bothers me is they throw about "This is a fixed point." "We know this so it has become fixed in our timelines" so much, so often, even within the same episode that they then ignore it in.

The Doctor perceives time in such a way as he can tell, fair enough, but how come he can then ignore and trick his way round it to save his own life, but not to work round this? How come knowing Reinette died without seeing him means he can't go back again, but Amy seeing herself and Rory on that hill top doesn't have to happen? They saw it for themselves? It just leave it all ringing hollow and leaves me with nothing really holding my interest. In much the same way the shifts in Voyager's character's personalities and everything else left Voyager being so unsatisfying.

You're assuming that Amy & Rory (or Amy alone) didn't go back to that hill during a time they weren't with the Doctor (or with him since it was in 2020, to fulfill the destiny) off screen.

NOTE: Had this page open, didn't see post above mine which basically says what I say.
Maybe the gaps in traveling for A&R were purposely put there by Moffat to explain this. He's shrewder than we think

There's nothing in the ep. to suggest that the graveyard scene was in the year 2020.

No one is saying it was. We know the graveyard scenes, just like the rest of the modern New York scenes were in 2012. We're just stating that Amy and Rory themselves were from after 2020.